eliel



Jan. 15, 1929. 1,699,136

' L. T. ELIEL THREE-PLATE METHOD OF AND APPARATUS FOR PRODUCING MAPS Filed May 4, 1925 s Sheets-Sheet 1 '6 6 W02.- 50 Til/EL. 5? I/Y ATTOR/EK Jan. 15, 1929. v 1,699,136

L. T. ELIEL THREE-PLATE METHOD OF AND APPARATUS FOR PRODUCING MAPS Filed May 4, 1925 5 Sheets-Sheet 2 5 Leo/v TEL/4.

Jan. 15, 1929. 1,699,136

L. 'r. ELIEL.

THREE-PLATE METHOD OF AND APPARATUS FOR PRODUCING MAPS Filed May 4, 1925 5 Sheets-Sheet 7 Jan. 15, 1929.

THREE-PLATE METHOD UP AND APPARATUS FOR PRODUCING MAPS Filed May 4, '1925 5 Sheets-Sheet 4 I 1% fim,

LED/v 75/51., 60 .35 6/ 25? v 82 41 ATTOR/S/El.

Jan. 15, 1929. 1,699,136

L. T. ELlEL THREE-PLATE METHOD OF AND APPARATUS FOR PRODUCING MAPS Filed May 4; 1925 5 Sheets-Sheet 5 LEO/v Til/EL,

Arrozgzxy:

determination of elevations and contour Patented Jan. 15, 1929.

UNITED, STATES PATENT OFFICE.

LEON '1. ELIEL, 0F PASADENA, CALIFORNIA, ASSIGNOR T0 ELIZABETH S. ELIEL.

THREE-PLATE METHOD OF AND APPARATUS FOR PRODUCING MAPS.

Application filed May 4, 1925. Serial No. 27,718.

This invention relates to topography, and more especially to a method of, and apparatus for, producing maps from aerial photographs; being more specifically directed to means for more accurately determining and describing elevations and contour lines.

The objects of my invention can be best understood if I first very briefly review the practice of map making by aerial photography as heretofore developed. The practice of flying an aeroplane over territory to be mapped and taking photographs with suitable camera equipment of successive sec tions of territory covered, and the subsequent cutting and iecing together of the developed pictures or making mosaic maps is well understood and in common use. One of the accomplishments unattained until recently, and only partially perfected, is the lines from aerial photographs.

As a primary development in the art of map making I disclosed and claimed in my application for United States Letters Patent filed July 12, 1923, Serial No. 651,069, a method of and apparatus for producing maps which contemplates the projection of successive pictures upon a plane surface, it superimposed effect being thereby obtained and contour lines being defined by the proper adjustment of the position of. and the focusing of, the pictures with reference to the position of the camera at the time of exposure. This invention referred to employs telaeroscopic means for determining and photographically recording the altitude and position from which each successive photograph was taken, from which data computation-could be. made for properly projecting the images obtained in a laboratory. The photographs used in this prior method were taken upon a single line of flight, each photograph, of course, having a common overlapping area with the photographs immediately preceding and immediately succeeding. The successive pictures were projected by a pair of projectors singly aligned to-correspond to the direction of the line of flight. This method is satisfactory in all but one respect, and that relates to the possibility of distortion upon the photographic plate due to deviation of the plate from the horizontal at the time of exposure; that is to say, there was no provision made for determining the amount of and correcting for tilt, except by making measurements on the ground covered by each picture and determining the tilt by graphical means or by elaborate computation from these measurements. Such determinations are often entirely impractical for the necessity of penetratin inaccessible country on foot and making tiese measurements on the ground nullifies the advantage of the aeroplane. While extreme precaution may be taken, such as using a gyroscopically mounted camera, the art of camera mounting has not developed to such a state that the camera is strictly unsusceptible to tilt, which may occur from an of several conditions such as the sudden urching of the aeroplane or from centrifugal forces introduced by imperceptable turns of the aeroplane. Variation from the horizontal of less than one degree will result in a considerable error when the uncorrected picture is reproduced upon a plane surface.

Thus, while photographs taken on a single line of flight may have been exposed in an approximately horizontal position there hasbeen heretofore no practical and accurate way to determine this slight tilt at the time of exposure. The'amount of this tilt is determined and correction is made possible by the apparatus and method hereinafter described wherein a third plate or photograph taken from a line of flight approximately parallel to a line intersectin the position of exposure of the first two photographs, or at least having an overlapping area with one or both of them, is superimposed in projection upon the first two ima es reproduced and the three plates adjuste until the resulting image projected from the three plates correctly defines the image of the contour selected.

Therefore:

An object of the invention is to provide a method and apparatus by which an accurate map may be produced from aerial photographs.

A further object is to provide in a method and apparatus of the class described means for selectively determining elevations and contour lines.

Another object is to afford means in a method and apparatus of the class described for determining and compensating for tilt of a photographic plate at the time of exposure; and to correct to ,a scale in the the accompanying drawings in which Fig, 1 is a diagrammatic representation of two lines of flight from which photographs are taken. I

Fig. 2 is a perspective view of the'appa ratus employed in-the laboratory in the projection of developed photographs.

Fig. 3 is a plan view of the apparatus em ployed. 3 V

Fig. 4 is a vertical section of one of the projectors employed, detailing the mounting %hereof and is taken on the line 4-4 of ig- 5 is a lan .view partly in section and partly in diagram of a flicker device.

Fig. 6 is a horizontal cross section of a projector'showing the plate holder in plan, being taken on the line 6-5 of Fig. 4.

Fig. 7 is an enlarged fragmentar view of a plate holder retaining means ta en on the line 77 of Fig. 6.

Fig. 8 is an enlarged fragmentary section of the plate holder adjusting means, being taken on the line 8-8 of Fig. 6.

Fi 9 is a diagrammatic view illustrating t e principles of projection involved in the method herein disclosed.

Fig. 10 is a diagrammatic representation of an overlapping, or superimposed effect, of three plates employed.

, Thefirst step in the procedure involved is i the taking of photographs of the territory to be mapped. I

It-is necessary that the altitude and loc'ation of the plane at the time of exposure be known; also the distance of separation or interval between successive exposures. This may be accomplished by means. of telaeroscopic information, details of which were disclosed in my 00- ending a plication above referred to. Brie the te aeroscope is an mstrument by whic vertical and horizontal angles can ,be readily determined and photogra hically recorded; this being accomplis ed by telescopes which are turned to old the aeroplane at the cross hairs thereof so that the elevation and position of the aeroplane can be computed therefrom. iReferring to Fig. 1 it will be evident that as a result of the'photographic record produced by the telaeroscope recording camera, data will be obtained which shows the vertical angle of the aeroplane at each telaeroscope, one being position at A and a second at B, with reference to the first line of flight F, A B being a known baseline. The telaeroscope at the same time indicates the horizontal angle of the aeroplane with relation to the ground base line.

The ground base line A'B, which may or may not be The angles (land D are determined by the telaeroscopes, as are also the angles G and H. The difference in elevation of the telaeroscope stations A and B being known, it is evident that the distance AB is also known and that the dimensions of triangle AEB can be readily calculated since the two acute angles are known from the telaeroscope observations, and one side is a known base line. Therefore, vation the position of the point E for any exposure can be readily calculated; or, in other words, the proper'position in the map for the vertical point of each of the photographs taken by the camera from the aero plane can be accurately and readily determined. t

It will be evident, therefore, in connection with each of the photographs taken by the camera in the aeroplane, that by a slmple series of calculations, a known height above the reference planeand the vertical point at each photograph in the reference plane can be determined.

In the three plate method it is necessary that a photograph be taken on a line of flight F, which is preferably parallel to the 11118 of flight F, but the photograph may be any one having a common overlapping area with one or both of the photographs taken on the line of flight F. It should, however, be so disposed that a triangle with good intersections is formed by the three points of exposure. i

A duplication of the telaeroscopic method altitude and position may be employed, alt ough, of course, any other method giving the same information would be equally satisfactory. 7

Assume for future reference that photographic exposures have been made at points of determinin by means of telaeroscopic obserhorizontal, is of a known length.

P, Q and R which have a common overlapping areareadily defined, as illustrated in Fig. 10.

Y Having any twosuccessive plates and a third plate properl disposed with an area common to one of t e other two taken from known elevations and with the horizontal distances between the respective points of exposure, that is to say, the air base lines,

topographer. The assembled autopo rapher includes three projecting machines or projecting the image of the plates upon a plane surface, orients the plates. determines the angular position of the plate at exposureand corrects for any departure from verticality of the optical axis.

The apparatus herein illustrated is extremely simplified to avoid confusion in disclosure of the method employed. However, the necessary functions and elements the upper horizontal extension of the channel tracks is formed with gear teeth providing gear racks longitudinally of the tracks. The track 10 is mounted rigidly upon standards 16 which are supported by the table 12, and the track 11 is rovided with trunnion ends 17 and 18 w ich are adapted to rotatably'seat in bearings provided by collars 19 and 20 which are formed in the upper ends of standards 21. A portion of the peripheral surface of the annular end 17 isformed with gear teeth which are adapted to be engaged by a small pinion 22 keyed to the end of a stem 23 which is adapted to extend through the end of a projection from the collar 19 and to be operated by a thumb wheel 24. The purpose of this arrangement is to afford a means for rotating the projectors X and Y upon an axis hereinafter referred to as the axis of indeterminate rotation, which in this particular case extends through the optical center ofthe projecting lenses of the projectors X and Y.

All of. the projectors are identical, the only difference being in the method of mounting them. Projectors -X and Y are carried upon the track 11 and projector Z is carried upon a transverse mounting 25, which is provided on its upper face with a scale 26 similar to the scales l3 and 14.

The details of each projector and the mounting may be understood by referring to Fig. 4. Each projector comprises a substantially square housing 30 having a lower tapered portion 31 which corresponds 'to a bellows extension, below the tapered portion, the housing being formed as a hollow ball 32, which is adapted to be supported by, and revolve in, a seini-spherical housing 33. Below the hollow ball 32 is a short cylindrical extension 34 which is adapted to retain a flicker device 35.

A set screw 27 is threaded through the housing 33 and is adapted to bear upon the hollow ball 32 for the purpose of securing the projector in a desired position. In the upper end of the housing 30 is provided an electric light source 36 behind which is disposed a reflector 37 and forwardly of which is placed a ground glass 38 for the purpose of light diffusion.

Condenser lenses 39 are retained below the ground glass by a circular strap 40 provided with an annular flange 41 secured to the interior of the housing 30.

One side of the housing is formed with an opening 45, which affords access to the interior of the housing for the purpose of manipulating a plate holder 46, and the opening 45 may be closed when the projector is in use b a sliding door 47. At the lower end of t e sliding door 47 is secured a curved clip member 48 (see Fig. 4) and also a pivoted spring 49 which has an upwardly extending finger 50 adapted to be engaged by the clip 48 to deter the spring 49 from dropping to a vertical osition when the sliding door is raised. he spring 49 comprises a flatresilient leaf terminating in a series ot-corrugations, and is intended to aid in retaining the plate holder 46 in precise position for projection of the plate image.

The plate holder consists of a rectangular frame 51 provided with lateral tongues 52 which are adapted to slide in recesses 53 formed horizontally in a plate holder support 54 secured within the housing 30 and resting u on an angular support 55. The plateholcfier support 54 presents perpendicular faces 56 and 57 which may be employed as locators for the proper disposition of the plate holder, and when the plate holder is pushed into the plate holder support 54 a pair of leaf springs 58 secured in a recess parallel to one of the recesses 53 resiliently urge the plate holder against the surface 56, and when the sliding door 47 is lowered the action of the springs 49 yieldably urges the plate holder against the surface 57. The plate holder may thus be always retained in the correct position, and if the plate carried by the holder is centered in the holder a correct projection may be made.

The means for adjusting or centering the plate 60 consists of 4 set screws 61, 62, 63 and 64 disposed one on each side of the rectangular plate holder and threaded horizontally through the plate holder so that each screw may bear upon the edge of the plate 60. It will be noted that a section of the plate holder 46 resembles an L, thus providing a resting surface 65 upon which the plate may be disposed and a vertical extension 66 through which the set screws may be threaded.

lib

30 upon t The frame defined by the inner faces 67 of the plate holder 46 is adapted to accurately register with an image frame on the plate 60. At the time of exposure the border 5 68 of the plate should be properly masked in the camera so that a transparent border frame will be produced, the diagonals of which would intersect at the geometric center of the image. As this border frame is the exact size of the frame presented by \the plate holder the plate may be properly centered in the plate holder by manipulating the set screws 61 to 64, inclusive, at the same time holding the plate and plate holder before a source of light to determine whether any lines of light are permitted to pass between the frame of the plate holder and the image frame of the plate. When the plate is roperly centered in the holder no light will e visible through the margins of the plate. A projecting lens 70 is properly mounted so that its optical center is concentric with the, hollow ball 32.

The spherical member 33 extends hori- 5 zontally from a. sliding support 71 which is adapted to enclose the track .11, and to slide 35 Set screws 75 maybe employed for securing the rojectors at any given position 1on- H gitudina lyof the track 11. The sliding support extending from the ball and socket joint of the rojector Z is adapted to be moved 40 longitu inally of the support or mounting 25, that .is to say, lateral y of the tracks .10 and 11, and may be finely adjusted by a pinion operated on 'a gear rack provided in the lower face of the support 25. The sliding support 76 adapted to longitudinal movement on the track 10 is-identical. with the sliding support 71 with the exception'that in place of a spherical support 33 extending from the support 71 the transverse track gupport 25 is carried by the sliding support The flicker device 35 carried on the lower end of the cylindrical extension 34 comprises a housing 80 having an opening 81 very 55 slightly greater than the diameter of the extension 34. A shutter 82 adapted to com- P pletely cover the opening 81 when swung below said opening 1s supported by an arm 83, which is pivoted at 84 and is adapted to so be-actuated by a solenoid 85 whose core 86 engages the arm 83. The pivotal connection between the core and the arm is very close to'the pivotal mounting 84 of the arm so that the pulling of the arm 83 is sufficient to as swing the shutter 82 from one side of the housing to the other. A coil spring 87 secured at one end to the core of the solenoid and at the other end to the housing 80 isadapted to work in opposition to the action of the solenoid.

When using the flicker device it is desired that the three should I be operated successively, that is to say, while the shutters will be normally retained open by the springs 87 it is desired that each shutter should be momentarily closed, one after the other, when in o eration. For this purpose I have provi ed a commutator contact arrangement diagrammatically illustrated in Fig. 5. A shaft 90 of a motor 91 is adapted to rotate a commutator 92 having a contact point 93 upon its peripheral surface which is adapted to successively engage three contact members 94. Each one of the contact ,members 94 leads by a wire 95 to a solenoid 85 and a wire 96 leads from the opposite end of the solenoid to a suitable source of eloctricenergy herein illustrated as a battery 97. The rotation of the commutator 92 will cause the contact portion 93 to successively engage the contact members 94, thus completing the circuit through the solenoids of successlve flicker devicesfand accomplishingI the flicker effect desired.

pon the rigid tertiary table 12 is mounted a secondary table 100, which is carried upon transverse tracks 101 and is adapted to be. moved longitudinally of the tracks and transversely of the table 12 b a conventional worm arrangement, the etails of which are not herein shown, the movement being accom lished by rotation of the thumb wheel 102. be secondary table is provided with longitudinally extending tracks 103 upon which a double primary table 104 is adapted to slide, being moved thereon. by a conventional pinion arrangement, the details of which are not herein shown, and which may be operated by a thumb wheel 105. The primary table 104 is provided with cross lines 106 and 107, which may be used for orienting the pictures projected, and for following contour lines or locating points, if desired. The primary table 104 comprises twothicknesses, identical in outline, the upper thickness being pivoted at the underside to swing upon an axis per endicular to the intersection of the cross lines 106 and 107. It is upon the upper thickness of this primary table 104 that the picture images are re ected.

A plotting table'108 is extended from the tertiary table 12 and ,is provided with a pantograph 109 carrying a tracin instrument 110. An am 111 extends rom the pantograph proper and is secured to the primary table 104, the movement of which is adapted to actuate the plalntograph for describing contour lines w 'ch appear in the projected images.

Referring to Figs. 9 and 10 I will now proceed to a description of the method employed in the use of the apparatus just described.

Three plates are used which I will designate as plates P, Q and R. Plates P and Q may be successive plates taken from the aeroplane at a distance appearin as PQ on the line of flight F, which will ereinafter be referred to as the air base line, and plate R is taken possibly from the next overlapping row so as to have an overlapping area common with plate P or Q, or both. Any two plates with a common overlapping area give all information necessary to map different common areas, with the exception of the angle of the plateat exposure. The thiril plate permits the determination of this ang e.

Plates P and Q are first placed in projectors X and Y, in which the flicker devices 35 are caused to remain open for the purpose of making the necessary adjustments. The images will be at approximately the same scale by virtue of the point of exposure having been at the same elevation.

It is next necessary to provide for orientation of the two plates. The two plates are adjusted so that the projection of their geometric centers falls on the longitudinal cross line 107 of the primary table 104. The two projectors X and Y are at this time preferably longitudinally separated so that the images do. not overlap on the table 104. The plates are then adjusted about the vertical axis by rotating the projectors in the ball and socket joints so that the projected images extend very nearly in the same direction.

A readily distinguished object is then picked out which appears in both plates, and the table 104 is moved until the selected object is projected from one plate on the line 107. The other plate is then turned by a slight rotation of the projector on the vertical axis until the projection of this selected object from the other plate also strikes the longitudinal cross line 107. It is sometimes necessary to make several adjustments on several different points of the plates before accurate orientation canbe produced. In.

other words, the two images of each object when the plates are properly oriented and otherwise corrected as to altitude and angle of exposure will always fall in a longitudinal line, that is, in a line parallel to the line connecting plate centers.

However, the orientation as described cannot beaccurately accomplished until the plates have been corrected for tilt, for, in fact, these two adjustments are interdependent andmus't be accomplished together.

An error in orientation may be discerned from an error in tilt, for if the error is in orientation only a slight rotation of the plates will cause the lines connecting the corresponding images of all common points to become parallel. If a tilt is resent it will be at once apparent for it Wlll be impossible to make these lines parallel by orientation.

After orienting as closely as possible, the plates P and Q are moved together to a plate base line distance compluted to give any desired contour line. T at 'this ma be accomplished can be understood by reference to Fig. 9. At a given distance of separation of the plates P and Q an image projected through the lens L will intersect only upon a single horizontal plane.

That is to say, if the image I (being of apoint taken on the horizontal plane AB) were projected from theplates P and Q simultaneously and the plates were disposed at the time of projection in the identical position which they occupied at the time of ex osure, the intersection of the lines of lig t 1 will occur only upon the plane AB. Furthermore, the lines of light .to any object in a different contour plane such as the planes CD and EF will not intersect at the plane AB, but will show double images out of re istry, causing a blur, at this elevation. ow, the same principle applies when the images are projected in miniature at .the same proportion. Thus, if the lates P and Q are separated a distance to ring the images appearing on the contour plane AB into registry, ob ects appearing in any other elevation such as CD or EF will not intersect and will appear as a blur. Now the points of various elevations may be brought into registry by either varying the .elevation of the plane surface upon which the images are pro'ected, such as lowering the table to CD or F, or by moving the plates P and Q closer together in order to obtain registry of the images of lower elevation. Thus, it will be seen that the contour. points of any elevation may be brought into registered projection by adjusting the distance of separation of the plates.

The flicker device is set in operation and the resulting image observed. It will be apparent that in the flicker effect obtained, only the lines of a contour which intersect at the particular elevation for which the setting is made appear to be in focus, and consequently will not be dlsturbed lg the alternate flicker of the two projectors and Y, while all portions of the projecting images which do not register, that 1s, which are not on the contour chosen, will appear as a blur wherein no object will be discernible.

While the flicker devices 35 are extremel useful in the three plate method of this invention, they may also be used 1n 9. two

platemethod to produce an illusion in which contours become apparent in the overlapping portionsoi the images of these photo.- graphs, as set forth in my application No.

i 651,069, filed July 12, 1923, on method of producin maps.

In pro ucing contours by this method as described thus far, I have assumed that the plates were taken with the camera axis directly vertical. In practice this is seldom the case, and, consequently, it is necessary to determine theamount, and direct1on of this error which I have called tilt, and to provide correction for it.

It is a well established fact that a photograph of a horizontal surface taken with a tilt or inclined camera axis does not record the objects on this horizontal plane at a uniform scale. Tilt, by the laws of perspective introduces a variation in scale.

However, by placing the plate so exposed in a projecting camera, the ax1s of which is in the same angular position as was the one or both plates. Instead of a line of no motion there will be an area of slight motion.

The plates should again be rotated slightly to make certain that the orientation is as both plates.

close as possible. If this fails to produce a perfect line the tilt may be determined and corrected by trial and error inthe following manner. I

Swing the plates about the optical lens center, which may be readily done by virtue of the freedom permitted by the ball and socket mounting of the projectors, until the images appear to be more nearly merged while flickering. Next choose several points from the area of least motion. It is preferable although not essential to choose four such points and they should be readily identifiable on each plate. If convenient two, of the points should lie on opposite sides of the plate center line from the other two.

If the relative error in tilt is large the operator will see at a glance the approximate situation which is causing the discrepancy, for the images from the two plates will be out of register. One or both plates must be tilted to bring the images into aproximate superimposition. Then it the efttwo points of one of the plates are much further apart than the corresponding two points of the other plate, as observed under a slow flicker,'and the right two points are much'closer together, it is obvious at once that there is a tilt about the lateral axis in It is evident that one plate alone could not be tilted to correct to satismeat-ea faction, since such a tilt would again displace the whole image horizontally causing a large discrepancy between all points. Both plates mustbe tilted in the direction indicated by this observation until the left and right points in the two plates are both as close to superimposition as possible.

. Then flicker rapidly. If there is still some motion apparent in what was formerly the area of least motion, rotate the plates slightly to check the orientation. If this fails to produce sharply defined lines there is still some tilt correction necessary. Observe the four points to ascertain if they are still in the area of least motion. If any or all oit them are moving further While flickering than other points in the area of least motion, new points should be selected from this area. Flicker slowly. Perhaps now the further pair of points appear more widely separated in one plate than in the other, while the nearer pair appear close together. This indicates a relative tilt about the longitudinal axis and both plates must be tilted about this axis to a position where the area of least motion approaches closest to a line. Orientation is rechecked, a new set of points se-' lected, if necessary, and the process continued until the area of least motion is reduced to a line.

The plates P and Q, are nowproperly oriented and have no relative tilt. The only error which may still be present is one of equal angular displacement of'the two plates about the base line axis. In other words, if instead of a horizontal table there were a vertical table the same distance from the plate base line, both projectors could be rotated on the track 11 through 90 and in the resulting image the same line of no motion would be intact.

In fact, this same situation could be present in any plane tangent'to the plate base line axis, which in this connection may ap-- propriately be called the axis of indeterminate rotation.

Practically. speaking, the axis of indeterminate rotation of the projectors herein employed passes through the projecting lenses 70, and the bearing provided by the collars 19'and 2O adords means of rotation about the axis of indeterminate rotation. A false registry may occur at the intersection of the primary table with some tangent plane perpendicular to the vertical axis of the two plates if they lie incorrectly placed about this axis of indeterminate rotation. The operator seeing a registry from two plates, therefore, has no way by the'use of two plates to know the proper disposition about this axis.

It is to provide this correction that the third plate R is used.

Plates P and Q are left in the position where they produce a sharp line. No further adjustment will be necessary on them, excepting a rotation about the axis of indeterminate rotation or plate base line axis,

which may beaccomplished with plates P and Q maintained as a unit on their track, rotation occurring by turning the thumb wheel 24, which, through the stem 23, operates the pinion 22 on the threaded portion of the collar 19.

Plate R, after having been placed in its proper osition by reference to the data obtained rom .the telaeroscopes, or other instruments, and reduced to the proper proportions on the scales of the track 10 and the transverse support 25, is next approximately oriented with either plate P or Q. All adjustments will be more easily accomplished while usin projector Z with either projectors X or fi', but not both. Orientation of the plate R is accomplished with the longitudinal line 107 turned to intersect the image center of plate R with either P or Q. In order to determine where this image center is, it will be necessary to provide on each ball and socket a mark 88 on the-ball, which will register with the opening at the upper end of the socket or semi-spherical support 33 when the lense axis is vertical. Then the center of the image will represent the point vertically below the plate center for the purpose of orientation. After orienting proceed to eliminate the tilt between R and P, or Q, precisely as described already, with the exception that plate P or Q is only to be tilted by rotation on the axis of indeterminate rotation, and, of course, will carry the other plate with it. This adjustment is more easily made ordinarily than the adjustment between plates P and Q. Since plate P or Q can only move about the one axis,

it is merely necessary to carry the rotation I through the probable angle. of error trying gresses.

to make plate R register as the rotation pro- WVhen an adjustment is accomplished to the point where a trueline of no motion occurs the correct position has been obtained to compensate for all errors of tilt,

and true contours and other information may be obtained'from the three plates. If plate P were used with R it is advisable, as a final check, to flicker Q with R and then all three together. If there is no motion it is proof that all corrections have been properly made. That is to say, the corrections for orientation and tilt are accurate. If there is a discrepancy in this final line of no motion, or any other for that matter, a mistake has occurred some place in the system and it will be necessary to systematically check back through to discover it.

A slight modification of this procedure is necessary in special cases, such as a uniformly sloping plane where contours would be straight and parallel. In this case two points in one area of. least motion for a high contour would be used and two points from a lower area with the plate centersmoved closer together for a lower contour.

On each adjustment the plate centers would have to be moved twice to bring the respective sets of points into the area of least motion or contour line, as the case might be.

In precise work it may be preferable to replace the ball and socket by a gimbal mounting. With this arrangement it is also possible to introduce an are on each axis by means of which an les may be read so that one three-plate ad ustment'will suffice for all time, by making a permanent record of the angles. It may further prove desirable to include slow motion screws of conventional design to facilitate the handlingof thev rojectors for precise work.

T e infallibility of the method described may be explained in this manner. Cameras replaced into the positions of exposure at P and Q and used as projectors for the. developed plates would cause the rays of light to fall back exactly on the source from which they came. Refer to Fig. 9. Imagine a huge level table of adjustable height, covering the entire area photo raphed. Sharp images would only register For those points with the two rays of 1i ht intersecting at the table. All other points higher or lower would show double images, such as.

point, or series of points, from two plates might in unusual cases :fall into registry;

of! this axis superimposition could not occur and falseness of the registry would at once be disclosed by the third plate, for if the plate images upon which the distances from images to table, resulting from tilt, were too great they would enlarge too much, while if these distances were too short the enlargement would not be enough. That is to say, this false registry might occur from the equal slight tilt of each projector about a lateral axis. Both images in eifect would be moved closer together in case each projector were tilted toward the other throu h an equal angle, and a false registry mig t occur on a series of points. But on account of the tilt the light from two points of the series will have to travel farther to the table, hence the images of the two points will appear at too large a scale. The third plate image if in its correct position will not register because its projected image of the two points will be to true scale. If on the other hand the first two plates are tilted flicker process it is, therefore, essential that the projection from the two plates occur at the same scale, in the same plane. Since all photographs are exposed from the same elevation, or corrected to the scale of the same elevation, it is obvious that the points of the same scale and in the same plane must lie in a plane parallel to the plate base line axis. By taking the third plate on a plate base line axis making a large angle with the other axis there is only one situation where the three will simultaneously produce a line of no motion. .This situation occurs with all projectors replaced into the angular position of exposure and projected onto a horizontal surface, or surface parallel to a plane passing through the center of the projecting lenses, which is a horizontal plane; This, of course, presumes that the three plates were exposed from the same altitude or corrected for this error.

justing the separation of the hereinbefore outlined.

When the projected images have been 104 the operator may 'follow the contour lines with the intersection of the crosslines on the table 104 and the pantograph will reproduce a contour line such as 115 upon the plotting table 108. As before pointed out, any other contour lines such as 116 may be determined and plotted by properly adprojectors, as

There may be occasions in very rare ,instances where the data obtained in the plates P and Q, will be suflicient to correct for tilt. This may occur where photographs have been taken along the ocean shore, where it is obvious that a known contour line will appear, or in country replete with erosive formations wherein the strata are very regular and of known position. In these instances the correct tilt may be ascertained by adjusting the plates to register on the known contour. There is only one point at which the projected images of the known contour lines will perfectly register along their entire overlapping length. If the plates were in' any but the position at which they were during exposure the projected image, by virtue of the diagonal disposition of light rays would cause the superimposed image to fail of registry at the extremities of the known contour.

I claim as my invention:

1. A step in the art of map making/comprising: taking three photographs of terriposure of each photograph; projecting in 1 miniature of the arrangement of exposure the photographic images upon a common field; and determining by observation and adjustment of the superimposed images the exact angular position of each negative at exposure.

2. A step in the art of map making comprising: taking photographs of territory to be mapped having a common overlapping area; projecting the images from the photographs upon a plane surface; causing the projected images to superimpose so that image objects occurring at a given elevation approximately register; and perfecting said registration by determining and correcting for equal angular displacement of two of the photographs about an axis of indeterminate rotation by the use of a third photograph.

3. A step in the art of map making comprisingrtaking: photographs of territory to be mapped having a common overlapping area; rapidly and alternately projecting the photographic images upon a common eld; and determining by observation and adjustment of the superimposed images the exact angular position of each negative at exposure.

4. A step in the art of map making comprising: taking photographs of territory to be mapped having a common'overlapping area; rapidly and alternately projecting in miniature of the arrangement of exposure the photographic images upon a common field; and determining by observation and adjustment of the superimposed images the exact angular position of each negative at exposure. 1

5. A step in the art of map making comprisingi taking photographs of territory to be mapped having acommon overlapping area; determining'the point of exposure of each photograph; rapidly and alternately projecting in miniature of the arrangement of exposure the photographic images upon a common field; and determining by observa-v tionand adjustment of the superimposed images the exact angular position of each mining and correcting for equal angular dis placement of two of the photographs about an axis of indeterminate rotation by the use of a third photograph.

7. An apparatus to be employed in map making comprising: a plane surface adapted to receive projected images; a plurality of projectors mounted to project images upon a commonarea of said plane surface; means permitting individual rotation and adjustment of said projectors; and means independent of said last named means for simultaneously rotating a pair of said projectors on an axis parallel to and longitudinal of the plane surface.

8. An apparatus to be employed in map making comprising: a plane surface adapted to receive projected images; a plurality of projectors mounted to project images upon a common area of said plane surface; means permitting individual rotation of said project0rs;' means independent of said last named means for simultaneously rotating a pair of said projectors on an axis parallel to and longitudinal of the plane surface; and flicker devices mounted on each projector and adapted to interrupt the issuance of projected light rays.

9. An apparatus to be employed in map making comprising: a plane surface adapted to receive projected images; a plurality of projectors mounted to project images upon a common area of said plane surface; means permitting individual rotation of said projectors; means independent of said last named means for simultaneously rotating a pair of said projectors on an axis parallel to and longitudinal of the plane surface; flicker devices mounted on each projector and adapted to interrupt the issuance of projected light rays; a plate holder in each projector; and means for centering a plate in said holder.

10. In map making, the art which comprises; taking photographs of territory to be mapped, said photographs having a common overlapping area; projecting the images of said photographs upon a common field; adjusting two of said images into an approximately true relation with each other and, While maintaining said relation between said two images, adjusting said two images and a third of, said photographic images into an approximately true relation with each other and perfecting said relation by adjustmcnt of the three images into perfect registry, the relation thus set up between said three images causing the photographs in the projectors to occupy the same relative angular positions as held by said photographs when taken, and constructing a map from the resulting information.

11. A method of producing a contour map, consisting of projecting the images of overlapping photographs onto a surface and determining all objects in a given contour by rapidly and alternately projecting the images onto a common surface to determine the exact points of registr In testimony Whereo I have hereunto set my hand at Los Angeles, California, this 27th day of April, 1925.

LEON T. ELIEL. 

